Don't pass around renderTargetContexts from onFlush callbacks

The drawing manager was just grabbing an opsTask off of these contexts
anyway. Instead, the onFlushResourceProvider can just snag an opsTask
off the renderTargetContext and populate the drawing manager's list of
onFlushRenderTasks.

Bug: skia:
Change-Id: I3bdb48176364bbd6e5a34fab437c45ed77d6687f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/236760
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
19 files changed